Mass spectrometers are instruments used to analyze the mass and abundance of various chemical components in a sample. Mass spectrometers work by ionizing the molecules of a chemical sample, separating the resulting ions according to their mass-charge ratios (m/z), and then measuring the number of ions at each m/z value. The resulting spectrum reveals the relative amounts of the various chemical components in the sample.
One type of mass analyzer used for mass spectrometry is called a quadrupole ion trap. Quadrupole ion traps take several forms, including three-dimensional ion traps, linear ion traps, and cylindrical ion traps. The operation in all cases, however, remains essentially the same. Direct current (DC) and time-varying radio frequency (RF) electric signals are applied to the electrodes to create electric fields within the ion trap. These fields trap ions within the central volume of the ion trap. Then, by manipulating the amplitude and/or frequency of the electric fields, ions are selectively ejected from the ion trap in accordance with their m/z. A detector records the number of ejected ions at each rink as they arrive. Regardless of the particular technology of mass spectrometer used, before sample molecules can be analyzed they must be ionized by one of various methods.
Electron ionization (EI) is one common method for generating sample ions. In EI, electrons are typically produced through a process called thermionic emission from a filement. Thermionic emission occurs when the kinetic energy of a charge carrier, in this case electrons, overcomes the work function of the conductor. In a vacuum chamber of a gas analyzer, where there is little gas or air to conduct heat from or react with a filament, a current through the filament quickly heats it until it emits electrons. The electrons are accelerated, usually with a set of electron optics, towards the sample, which may be contained within a mass analyzer (e.g., an ion trap). As the electrons travel through the gaseous sample, the electrons interact with, fragment, and ionize molecules in the sample. The charged particles can then be transported and analyzed using additional electric fields.
EI uses relatively energetic electrons with energies of around 70 electron volts to ionize sample molecules, and as such can sometimes cause weaker molecules to fragment into smaller ions. For this reason energetic electrons are sometimes referred to as a “hard” ionization source. Fragmentation can be beneficial in cases where one wishes to learn more about the parent ion by analyzing the fragment or “daughter” ions. In cases where fragmentation is not desired (e.g., it is desirable to know the mass of the parent ion), a softer ionization technique may be appropriate.
One such soft ionization technique is photoionization (PI). In PI, a light source emits photons, generally in the ultraviolet wavelength range, to provide sufficient energy to eject electrons from molecules in the chemical sample, thereby ionizing them. The photons in PI have lower energy than the electrons in EI, typically 5-10 electron volts as opposed to the 70 electron volts typical of EI. As such, PI generally allows sample compounds to remain intact. Broadly speaking, PI can be accomplished by two different techniques: single-photon ionization, and multi-photon ionization. Single-photon PI occurs when the PI source produces photons that individually have sufficient energy to ionize molecules. This usually corresponds to about 10.6 electron volts, or 110-130 nm wavelength. In multi-photon PI, the photons have less energy, perhaps only 5 electron volts, or 240-260 nm wavelength, and therefore multiple photon-molecule interactions are required to ionize the molecule.
Single-photon PI generally requires a source such as a plasma lamp in the ultraviolet range. Traditional ultraviolet light sources are generally large compared to the dimensions of an ion trap, and may require the source to be located away from the trapping region. As a result, ions must be created outside of the ion trap and transported into the ion trap via the use of electric fields or fluid flow. However, creating ions outside of the ion trap may result in reduced sensitivity of the mass spectrometer, and the electron optics required to transport the ions may add additional complexity to the instrument. Also, some architectures used for mass analyzers that lend themselves to miniaturization, for example ion traps, may not be effective at efficiently trapping ions generated from an external source. In addition, the extra ionization chamber requires more space and larger vacuum pumps to evacuate, making it potentially unsuitable for applications where size and power consumption are an issue.
Laser diodes are small enough to provide an ultraviolet light source directly into an ion trap; however, they are limited to a wavelength of 248 nm, which corresponds to about 5 electron volts. This energy is insufficient for single-photon PI. Multi-photon PI is possible, however, with appropriate pulsing of laser diodes.
Embodiments of the disclosure described herein may overcome at least some of the disadvantages described above.